1. Field of the Invention
The present invention relates to an image display system, an image display apparatus and a calibration method.
2. Description of the Related Art
A color liquid crystal display forms a color image by controlling the luminance of a backlight, and the transmittance of each liquid crystal cell. The pixel value of an input image signal is represented, for example, by a value of 256 steps or levels, with respect to an R (red) component, a G (green) component, and a B (blue) component, respectively. A liquid crystal driving signal (drive voltage) is decided from the above-mentioned pixel value through a matrix operation or through reference to a look-up table, so that the liquid crystal transmittance can be controlled by providing the drive voltage to a liquid crystal driving unit.
The color liquid crystal display (hereinafter referred to as a calibration display), which is provided with a calibration function to be carried out by a personal computer, measures the luminance and the chromaticity of each of a plurality of color charts displayed on a screen. The calibration display adjusts arithmetic operation (i.e., computation) parameters and the look-up table for deciding a backlight driving signal (a drive electric current or a pulse width modulation duty ratio) and the liquid crystal driving signal. According to this, it is possible to manage so as to make smaller an error or difference between a color which should be displayed and a color which is actually displayed. With the general calibration display, a user sets one set of calibration desired or target values such as a white color luminance, a black color luminance (or a contrast ratio=white color luminance/black color luminance), a white chromaticity, a color gamut, a gamma coefficient, and so on.
The calibration display displays, at least, a color chart of three primary colors for adjusting the color gamut, a white color chart for adjusting white chromaticity, a black color chart for adjusting black color luminance, a white color chart for adjusting white color luminance, and a plurality of gray color charts for adjusting a gamma curve. The calibration display adjusts the arithmetic operation parameters and the look-up table values for deciding the backlight driving signal and the liquid crystal driving signal so as to meet the set calibration target values.
There is a display which divides a backlight into a plurality of light source blocks, and carries out variable control of the luminance of the backlight for each of the light source blocks. According to such a local dimming display, there will be obtained such effects as an improvement in the contrast ratio or a reduction in the power consumption. There are some display devices in which the luminance of an entire backlight is changed in a uniform manner according to an image signal to be inputted, without dividing alight source into blocks, but in the following description, such display devices are handled as a kind of dimming display (i.e., dimming with the number of light source blocks being 1).
The characteristics of the arithmetic parameters and the look-up table for deciding the backlight driving signal and the liquid crystal driving signal according to the image signal to be inputted vary with the luminance of the backlight. For that reason, in the case of local dimming displays in which the luminance of the backlight can be switched or changed over among a plurality of levels of luminance, it is necessary to carry out a calibration with respect to each of the plurality of levels of backlight luminance.
In the patent gazette of Japanese patent application laid-open No. 2006-145798, there is described a technique which, in a local dimming display, measures the luminance and the chromaticity of each of a plurality of color charts displayed on a screen, and adjusts arithmetic operation parameters and a look-up table for deciding a liquid crystal driving signal from a pixel value to be inputted.